Here is a simple way of doing it...
cls
set display mode 1280, 720, 32
sync on
sync rate 60
make object cube 1, 1
hide object 1
make object cube 2, 2
position camera 5, 5, -5
point camera 0, 0, 0
do
ink rgb(255, 0, 0), rgb(0, 0, 0)
line3d(-1, -1, -1, 1, 1, -1)
sync
loop
function line3d(x1, y1, z1, x2, y2, z2)
ln_x1 = scr_x(x1, y1, z1)
ln_y1 = scr_y(x1, y1, z1)
ln_x2 = scr_x(x2, y2, z2)
ln_y2 = scr_y(x2, y2, z2)
line ln_x1, ln_y1, ln_x2, ln_y2
endfunction
function scr_x(x, y, z)
position object 1, x, y, z
ret = object screen x(1)
endfunction ret
function scr_y(x, y, z)
position object 1, x, y, z
ret = object screen y(1)
endfunction ret
